Well random vertices is good enough. As long as they are distributed evenly around the whole volume, not based on mesh density. Which was a problem in first versions of vray proxy display in max afaik. Where, for example, since trees have more polys where leaves are, vertex/cloud display picked those vertices at first so you had to request very big amount of display vertices to see where trunk is placed.

As @lacilaci said problem with point cloud based on vertices is you can't actually see where triangles are, so medium-poly and low-poly objects are just mess and you can't see actual object shape. Besides it requires special rendering and coding. That's why suggested very simple solution from indigo render, where you have proxy linked to mesh, not object. This solution has some neat advantages:
- it doesn't require any coding of proxy display
- it's extremely fast as blender loads only manually decimated mesh, not full one and generating view
- after heavy decimation blender leaves edges on hi-poly objects, this looks really good on trees (screen attached)
- you can precisely decimate original mesh so it can even cast shadows
- you actually see where geometry is so it's way easier to put stuff into scene and avoid collisions
- you can use any geometry like billboarded view of tree using 6 triangles and get proxy display

This one is still heavy to display (10% of faces), but I keep both meshes inside external blend file so I can generate both original proxy and have decimated one. It requires a bit of work, but considering creation of whole asset it's very little.

I am working on multiple UV map and vertex color support for the Blender addon.

Nice!

However, I would not add the UV dropdown as a property of the image texture.
It is a property of the 2d/3d mapping, see viewtopic.php?f=5&t=1558#p17979
So it should be added to the 2d/3d mapping nodes (3d mapping should only show it in the UI if the mode is set to "UV").